Conventionally, coating paste or mix is applied to a paper or board web using so-called spray coaters in which a coating paste is sprayed on a paper or board web from a distance. The quality of the coating is controlled by adjusting the distance between the spraying nozzle and the web, the spraying jet velocity and the mass rate of spraying. Furthermore, the coat quality can be modified by adjusting the coating paste formulation and proportion of its components. In nozzles utilizing compressed air for atomizing the coating paste, the air pressure supplied to the nozzle provides an additional control variable. The spraying apparatus is typically enclosed in a tray hood which surrounds the nozzle assembly and extends over the entire cross-machine width of the web. The bottom of the tray is provided with a duct for removing excess coating paste. Because the coating paste being sprayed is an aerosol, it expands to fill the entire volume of the tray hood. However, the aerosol also forms larger droplets which may impair the coating quality if these larger droplets contact the web. Further, as the gaps between the coater and the web are awkward or difficult to seal, the coating paste aerosol typically escapes to the surroundings of the coater hood and may impair the quality of the coating applied to the web and may also soil equipment in addition to the coater. Removal of such excess aerosol by means of suction is difficult since separation of the paste droplets from the air flow prior to the coating paste recirculation requires use of expensive and complex filtering devices.
U.S. Pat. No. 4,944,960 describes a spray coater utilizing the above-described suction methods for reducing the dessemination of the excess coating aerosol.